Base body of reflecting mirror and method for preparing the same

ABSTRACT

A light-weight base body of a reflecting mirror, such as those used in reflecting astronomical telescopes, is proposed which is made from fused silica glass or high-silica glass and is advantageous in respect of the excellent thermal and mechanical stability in dimensions to ensure high performance of the reflecting mirror. The base body is composed of a front plate, i.e. a surface plate to provide the optical surface, and a supporting body of porous foamed glass integrally bonded to the front plate. These two parts of the base body can be bonded together by sandwiching a layer of a finely divided silica powder therebetween and heating the assemblage at a temperature higher than the softening point of the silica powder so that the silica powder is softened or melted to firmly join the two parts sandwiching the powder layer. The base body can be further improved in respect of the mechanical stability by providing a rear plate backing the porous foamed body and a reinforcing hoop-like side layer surrounding the side surface of the porous foamed body, each made from fused quartz glass or high-silica glass and bonded to the porous foamed body by utilizing melting of a layer of silica powder therebetween.

This is a divisional application of Ser. No. 07/775,095 filed Oct. 11,1991.

BACKGROUND OF THE INVENTION

The present invention relates to a base body of a reflecting mirror anda method for the preparation thereof. More particularly, the inventionrelates to a base body of a large-sized reflecting mirror used inreflecting astronomical telescopes and for collimation or diffusion oflight beams, which is characterized by outstandingly light weight andstill is free from any adverse influences on the dimensional precisionand accuracy of the mirror surface due to mechanical deformation by theweight of the body per se and changes in the ambient temperature, aswell as to a method for the preparation of such a base body of areflecting mirror.

Reflecting mirrors in the prior art used in astronomical telescopes orfor collimation or diffusion of light beams are prepared by lapping andpolishing a surface of a mirror base made from fused quartz glass orhigh-silica glass to have a surface with flatness or a specifiedcurvature of high precision, and providing the thus polished surface ofthe base body with a reflecting layer of a metal such as aluminum by themethod of, for example, chemical vapor deposition at a temperature of400° to 800° C. to give a reflecting surface. Such a reflecting mirroris used usually by being mounted on a supporting stand in a movable orrotatable fashion to facilitate taking a desired disposition. It isessential for the base body of a reflecting mirror that the base bodyhas such physical properties that the accuracy of the mirror surface isnot affected by various outer conditions such as changes in the ambienttemperature to cause thermal expansion or shrinkage and changes in thedisposition of the mirror to cause mechanical deformation of the basebody by gravity.

When the reflecting mirror is relatively small, for example, having adiameter off 20 cm or smaller, the above mentioned requirements for themirror base can be readily satisfied. Along with the recent trend thatreflecting mirrors of larger and larger size, for example, having adiameter of 1 meter or even larger are demanded with an object toenhance the efficiency of the mirror system, the above mentionedrequirements for the mirror base can be satisfied with increasingdifficulties to ensure high accuracy of the reflecting surface. Namely,other than the high temperature at which the vapor deposition of themetal layer for the reflecting surface is performed, even a very slightchange in the temperature of the mirror body caused by the changes inthe ambient temperature and by the irradiation with high-energy lightbeams may cause a great thermal expansion of the base body so that themirror surface is sometimes subject to warping or undulation resultingin a decrease in the performance of the reflecting mirror. This is thereason that the mirror base is formed from fused quartz glass orhigh-silica glass having an outstandingly small thermal expansioncoefficient.

Besides the above mentioned thermal expansion or shrinkage, anotherserious problem in a large-sized reflecting mirror is the mechanicaldeformation of the mirror base, because a large-sized reflecting mirrornaturally has a large weight, so that the mirror base is under a greatinfluence of the gravity to cause deformation of the mirror base indifferent ways as the disposition of the mirror is changed by beingrotated or moved on the supporting stand. Accordingly, various attemptsand proposals have been made in the prior art for decreasing the bodyweight of a reflecting mirror by the improvement of the structure of thebase body of the mirror without sacrifice in the mechanical strength asa support of the reflecting surface, to comply with the practicalrequirement to ensure good operability of a large-sized reflectingmirror having a glass-made mirror base.

For example, Japanese Patent Publication 63-57761 discloses alight-weight glass-made base body of a reflecting mirror forastronomical telescopes, which consists of a front plate, i.e. thesurface plate for forming the reflecting surface by metal platingthereon, a rear plate or backing plate as a base for supposing the frontplate and a latticework therebetween composed of a plural number of rowsof pipes made from fused quartz glass. In the latticework of pipes, eachpipe of the pipe rows is contacted in a cross-stitch arrangement withthe two pipes in the respective adjacent rows forming contacting linesor contacting zones while the wall thickness of the pipes is smalleralong the above mentioned contacting lines or zones than in the otherportions of the pipe walls and the pipes are joined together into anintegral latticework by welding along the contacting lines or zones.Such a complicated latticework structure of the intermediate layerbetween the front plate and the rear plate of the base body, however, isindustrially very disadvantageous because of the very large costs forthe preparation thereof. In addition, the mirror base having such alatticework structure has poor mechanical strength in the directionwithin the surface plane so as not to withstand the high-precisionlapping and polishing works of the optical surface, before plating witha metal, to have a desired flatness or curvature of the reflectingsurface.

Moreover, it is a very difficult matter to obtain the pipe elementsforming the latticework having an exactly equal effective height so thatthe front plate after polishing supported by the latticework unavoidablyretains a strain corresponding to the height difference in the pipeelements forming the latticework to cause deformation or undulation ofthe reflecting surface after lapse of a certain length of time. Therigidity of such a latticework is of course inherently anisotropic anddiffers between the directions perpendicular to and parallel with thereflecting surface, so that the reflecting mirror having such a basebody can hardly be used when the mirror must take different dispositionsby being rotated or moved on the supporting stand due to the pooraccuracy of the reflecting surface when the disposition of the mirror isvaried.

Further, Japanese Patent Publication 61-26041 discloses anotherlight-weight glass-made base body of a reflecting mirror forastronomical telescopes. The base body of fused quartz glass alsoconsists of a front plate, a rear plate and an interposed latticeworklayer therebetween integrated into a body by welding. The latticework isprepared by putting plate-formed and/or tubular lattice elements on asupporting plate to form a lattice and filling the spaces formed betweenor surrounded by the lattice elements with tiny pieces of the same glasssusceptible to sintering, followed by sintering of this assemblage asfastened with a graphite ring in a furnace under a non-oxidizingatmosphere. The thus prepared latticework is sandwiched between thefront plate and the rear plate and welded together into an integral basebody to be finished by polishing the surface of the front plate. Such abase body of a reflecting mirror is industrially disadvantageous and notpractical due to the very lengthy and troublesome procedure ofmanufacture, with consequently very high costs, in addition to theproblem that the front plate bonded to the latticework by weldingretains substantial strains at the welded portions to greatly affect thedimensional accuracy of the reflecting surface.

SUMMARY OF THE INVENTION

The present invention accordingly has an object to provide a novel basebody of a reflecting mirror having an outstandingly light weight andstill having excellent stability against mechanical as well as thermalchanges in its dimensions to ensure good operability and highperformance of the reflecting minor prepared by metal-plating on thepolished surface of the base body, as well as to provide a method forthe preparation of such a base body of a reflecting mirror.

Thus, the base body of a reflecting mirror provided by the invention isan integral body comprising:

(A) a front plate having an optically flat or curved surface made fromfused quartz glass or high-silica glass; and

(B) a porous foamed body of fused quartz glass or high-silica glassbonded to the surface of the front plate opposite to the optically flator curved surface.

It is preferable that the porous foamed body bonded over the wholesurface to the front plate has a bulk density in the range from 0.1 to1.1 g/cm³ and the porosity thereof mainly consists of closed cells or,more preferably, at least 15% by volume of the porosity consists ofclosed cells.

The above defined base body of a reflecting mirror is prepared by themethod comprising the steps of:

(a) laying a front plate having an optically flat or curved surface madefrom fused quartz glass or high-silica glass and a porous foamed body offused quartz glass or high-silica glass one on the other, with thesurface of the front plate opposite to the optically flat or curvedsurface facing the porous foamed body, with an interposed layer of afinely divided silica powder therebetween; and

(b) heating the assemblage of the front plate and the porous foamed bodysandwiching the layer of the finely divided silica powder at atemperature higher than the softening point of the silica powder so asto integrate the front plate and the porous foamed body.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a perspective view of the base body of a reflecting mirroraccording to the invention, of which the optical surface of the frontplate is flat, and FIG. 2 is a radial cross sectional view of the same.

FIG. 3 is a radial cross sectional view of a base body of a reflectingmirror as another embodiment of the invention in which the base body hasa rear plate integrated thereto.

FIG. 4 is a radial cross sectional view of a base body of a reflectingmirror as a further embodiment of the invention in which the front platehas a concavely curved optical surface and the base body has a rearplate and a side-reinforcing layer of glass integrated thereto.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As is described above, the essential components forming the inventivebase body of a reflecting mirror include (A) a front plate having anoptically flat or curved surface made from fused quartz glass orhigh-silica glass; and (B) a porous foamed body of fused quartz glass orhigh-silica glass bonded to the surface of the front plate opposite tothe optically flat or curved surface.

The front plate is a plate on which a highly reflective layer of a metalsuch as aluminum is formed by the method of vapor deposition to give anoptical reflecting surface. Therefore, the front plate must have asurface having optical flatness or desired curvature depending on thetypes of the mirror which may be concave or convex having a specifiedfocal length in compliance with the intended use of the reflectingmirror. The front plate is made preferably from transparent fused quartzglass having as high as possible purity or a purity of at least 99% byweight. When it is made from high, silica glass, the content of silicondioxide in the high-silica glass is desirably at least 70% by weight.The thickness of the front plate naturally depends on the size of thereflecting mirror, and such thickness is desirably larger in a frontplate having a larger diameter in order to ensure good mechanicalstrength although an excessively large thickness thereof is undesirabledue to the increased weight so as not to meet the requirement for alight-weight mirror base. It is of course desirable that the glassforming the front plate is free from any bubbles and has hightransparency. The thickness also depends on the thickness of the porousfoamed disc body to which the front plate is bonded all over thesurface. For example, the thickness of the front plate is in the rangefrom 2% to 20% of the total thickness of the front plate and the porousfoamed disc body assuming that the porous foamed body, has a form of aboard or slab having two parallel surfaces with a uniform thickness.

The above described front plate of fused quartz glass or high-silicaglass is bonded to a porous foamed disc body of also fused quartz glassor high-silica glass of equally high purity. The porous body should havea bulk density in the range from 0.1 to 1.1 g/cm³, and at least 15% or,preferably, at least 30% or, more preferably, at least 60% of theporosity thereof is provided by closed cells. The volume fraction of theclosed cells in the overall porosity of the porous body can be readilydetermined from the values of the bulk density, true density of silicaor high-silica glass forming the matrix of the porous body and volume ofthe open cells determined by immersing the porous body in a liquid suchas water. When the volume fraction of closed cells is sufficiently high,the closed cells may form a three-dimensional network structure which isanisotropically highly resistant against outer forces in everydirection. When the bulk density of the porous body is too small, themechanical strength of the porous body would be unduly low so that noreliable support can be provided as a base of the front plate to havethe reflecting surface. When the bulk density thereof is too high, onthe other hand, the body weight of the base body is naturally too largeso as not to meet the requirement for decreasing the weight of alarge-sized reflecting mirror. The closed cells should desirably have adiameter in the range from 0.01 mm to 3 mm. When the closed cells aretoo coarse, the mechanical strength of the porous body would bedecreased, while a porous body formed from too fine closed cells cannotbe light enough to meet the requirement for a light-weight base body.The porous foamed body is usually in the form of a disc or slab havingupper and lower surfaces parallel to each other though not limitativethereto. For example, the upper surface can be inclined relative to thelower surface depending on the particular fashion of installation of thereflecting mirror in the optical instrument. When the porous foamed bodyis in the form of a disc or slab, the thickness of the porous foamedbody is not particularly limitative but it is usual that the thicknessthereof is in the range from 80% to 98% of the total thickness of thebase body.

The porous foamed disc body of fused quartz glass or high-silica glasscan be prepared according to a procedure known in the art. For example,a powder of fused quartz glass consisting of silicon dioxide havinghydroxy groups is heated in an atmosphere of ammonia to be reactedtherewith followed by shaping into a desired form and sintering.Alternatively, a powder of fused quartz glass is first shaped into aform and sintered and the sintered body is then ammoniated by heating inan atmosphere of ammonia. Thereafter, the ammoniated sintered body isheated in an electric furnace at a temperature of 1500° to 1800° C. tocause softening or melting of the ammoniated silicon dioxide which isexpanded by the gas evolved from the glass to give a porous foamed bodyof which the porosity mainly consists of closed cells. Furtheralternatively, a foamed porous body of glass can be prepared by heatinga blend of a glass powder and a blowing agent at a temperaturesufficiently high to cause decomposition of the blowing agent to evolvea gas and to cause softening of the glass powder. At any rate, it isimportant in these processes that the conditions of foaming should beselected so as to obtain closed cells having an adequate diameter and toprevent predominance of open cells by excessively increasing thetemperature.

The porous foamed body of glass obtained in the above described manneris then cut into a desired form such as a circular disc or square orrectangular slab depending on the size and form of the reflecting mirrorto be prepared therefrom. The porous foamed body of glass is bonded, onone surface, to the front plate of transparent fused quartz glass orhigh-silica glass to provide the optical surface. Accordingly, it isnecessary that the surface of the porous foamed body is shaped to have aform capable of being contacted with the surface of the front plateopposite to the optically flat or curved surface as closely as possibleso that they can fit each other over substantially their whole surfacesas completely as possible.

In the bonding work of the front plate and the porous foamed disc bodyas the support, they are laid one on the other with an interposed layerof a finely divided silica powder having a uniform thickness of, forexample, from 1 to 3 mm formed by spreading the powder all over thesurface, or in an amount of the freely divided silica powder spread overthe surface in the range from 2 to 200 g/m² or, preferably, from 30 to100 g/m². When the thickness of the silica powder layer is too small, nocomplete bonding can be obtained between the front plate and the porousfoamed disc body while, when the thickness is too large, some decreaseis caused in the bonding strength. The silica powder should have anaverage particle diameter as fine as possible or, preferably, notexceeding 10 μm. The silica powder should have a softening point lowerthan that of the porous foamed body of glass, preferably, by 50° to 100°C., or the softening point of the silica powder should be in the rangefrom 1550° to 1800° C. or, preferably, from 1600° to 1700° C. When the,softening point is higher than that of the porous glass body, the porousfoamed disc body may cause deformation or may be subject to bursting ofthe closed cells before the silica powder layer is softened. In thisregard, silica powders having a specific surface area of at least 5 m²/g or, preferably, at least 20 m² /g should be used. Silica powdersproduced by the so-called sol-gel method are suitable. In particular,so-called fumed silica and precipitated silica fillers having a specificsurface area of, for example, at least 50 m² /g, which can be softenedusually at 1400° to 1700° C., are quite satisfactory for this purpose.

The thus obtained assemblage of the front plate, porous foamed disc bodyand interposed layer of the finely divided silica powder is then mountedon a surface plate of, for example, graphite and heated at a temperaturehigher than the softening point of the silica powder but lower than thesoftening point of the porous body of glass for a length of time of fromabout 1 to about 4 hours under pressing by mounting a suitable weightof, for example, graphite thereon, so that the silica powder is softenedor melted to act as an adhesive between the front plate and the porousbody. Since the thickness of the silica powder layer is so small, it isusual that the molten silica powder is absorbed by the porous glass bodyso that substantially no layer of the molten silica powder as anadhesive can be found at the interface between the front plate and theporous body after the bonding treatment. It is desirable in the thusbonded interface that the effective bonding area at the cell walls is atleast 5% or, preferably, at least 20% of the overall apparent bondingarea, the balance being the areas of the pore spaces of the cells in theporous body.

The thus prepared composite body, which is illustrated in FIG. 1 by aperspective view and in FIG. 2 by a radial cross sectional view,consisting of the front plate 1 and the porous foamed disc body 2, hashigh mechanical strength suitable for the lapping and polishing works ofthe optical surface prior to plating of the optical surface with a layer3 of a metal such as aluminum and silver to form a reflecting surface,If desired, another plate 4 of fused quartz glass or high-silica glass,called a rear plate or backing plate, can be bonded to the surface ofthe porous foamed disc body 2 opposite to the front plate 1 as isillustrated in FIG. 3 by a radial cross sectional view, so that thecomposite body can be imparted with further increased mechanicalstrength. The quality or purity of the fused quartz glass or high-silicaglass forming the rear plate 4 need not be so high as in the front plate1, and presence of a small number of bubbles or some opacity has noparticular adverse influences. The method for bonding of the rear plate4 to the porous foamed disc body 2 can be substantially the same as inthe bonding work of the front plate 1 to the porous body 2.

As is illustrated in FIG. 4 by a radial cross sectional view, the mirrorbase illustrated in FIG. 3 can be further provided with a hoop-likereinforcing layer 5 having a thickness of, for example, 1 to 5 mmsurrounding and bonded to the side surface of the porous foamed discbody 2. The reinforcing layer 5 is made from fused quartz glass orhigh-silica glass of a quality which can be about the same as that ofthe rear plate 4. A convenient method for providing such a hoop-likereinforcing side layer 5 is as follows. Thus, a hoop of fused quartzglass having an adequate wall thickness and diameter is prepared byradially cutting a pipe of fused quartz glass. The inner diameter of thehoop should be somewhat larger than the diameter of the porous foameddisc body to be put into the hoop before bonding so that, when theporous foamed disc body is put inside the hoop, s small gap having awidth of, for example, 0.5 to 2 mm is formed around the porous foameddisc body. This annular gap is then filled with a finely divided silicapowder such as that used in bonding of the front plate and rear plate tothe porous foamed disc body.

The bonding works of the front plate, rear plate and hoop-likereinforcing side layer can be performed in one step. A typical proceduretherefor is as follows. In the first place, the front plate is placed ona horizontal surface plate of graphite and the finely divided silicapowder is spread over the front plate to form a uniform layer of thepowder. Then, the porous foamed disc body is put on the layer of thesilica powder and further the silica powder is spread over the surfaceof the porous foamed disc body to form a second silica powder layer onwhich the rear plate is mounted. Then, the reinforcing hoop is put tosurround the porous foamed disc body. The gap formed between the porousfoamed disc body and the hoop is filled with the silica powder. Aftermounting a graphite weight on the rear plate, the assemblage isintroduced into a furnace and heated there at a temperature to causesoftening of the powder layers so that the four parts, i.e. the porousfoamed disc body, front plate, rear plate and reinforcing hoop, areintegrated into a base body of a reflecting mirror. It is usual that adecrease is caused in the thickness of the porous foamed disc bodyheated under a graphite weight so that the width of the reinforcing hoopshould be somewhat smaller than the thickness of the porous foamed bodybefore heating under the graphite weight.

In the following, the present invention is illustrated in more detail byway of examples and comparative examples.

Example 1

A carbon mold was filled with a finely pulverized silica powder of 98%purity containing about 300 ppm of hydroxy groups and having a particlediameter not to exceed 100 μm. The powder was heated at about 1400° C.for 1 hour to prepare a sintered body which was then heated in anatmosphere of ammonia gas at 800° C. for 6 hours to effect theammoniation reaction, and then heated at 1500° C. for 2 hours so thatthe sintered body was softened and expanded by the gas evolved therefromto give a porous foamed body of fused quartz glass having a bulk densityof about 0.4 g/cm³. The volume fraction of closed cells in the overallporosity was about 61%. The closed cells had diameters ranging from 0.08to 0.8 mm. The porous foamed body thus obtained was cut and shaped intoa disc having a diameter of 500 mm and a thickness of 8 mm.

The thus prepared porous foamed disc body having flat surfaces wasmounted on a fused quartz glass plate having a diameter of 500 mm and athickness of 0.5 mm as a rear plate put on a surface plate of graphite.A layer of a finely divided silica powder was interposed between theporous disc body and the rear plate. This powder layer was formed byspreading a fumed silica filler having a specific surface area of about50 m² /g (Aerosil 50, a product by Nippon Aerosil Co.) all over thesurface, and the layer had a thickness of about 1 mm after gentlestamping. Further, another plate of high-quality, transparent fusedquartz glass as a front plate was mounted on the porous disc body alsowith an interposed powder layer of the same fumed silica filler in athickness of about 1 mm after gentle stamping. The amount of the silicapowder spread over the surface was 80 g per m² of the surface. The thusprepared assemblage of the rear plate, porous foamed disc body and frontplate on the surface plate with interposed layers of the silica fillerwas heated under a load of a graphite-made weight of about 12 kg at atemperature of about 1400° C. for 50 minutes so that the rear plate,porous foamed disc body and front plate were bonded together into anintegral base body having a thickness of 9 mm and a diameter of 500 mm.

The base body obtained in the above described manner was evaluated formechanical strength and stability by the testing procedures describedbelow.

Test 1

The base body was mounted on a horizontal surface plate with the frontplate facing upwardly and a weight of 500 kg was placed on the centralcircular zone of 50 cm² area and kept as such for 3 minutes at roomtemperature to measure the depression at the center of the body underweight and the residual depression after the weight was removed. Theresults were that the depression at the center under load was 0.4 μm andsubstantially no residual strain was found after removal of the 500 kgweight.

Test 2

The base body was horizontally supported at two radially opposite pointsand kept at room temperature without mounting a weight to measure thedepression of the plate at the center by the body weight of the plateper se. The result was that the depression at the center was 1 μm.

Test 3

The base body was horizontally supported in the same manner as in Test 2and an increasing weight was mounted on the central circular zone of 3cm² area to record the relationship between the weight added anddepression of the plate at the center as well as the weight when theplate was broken. The result was that the depression at the center ofthe plate was 0.1 mm, 0.2 mm, 0.4 mm and 2 mm under the load of 0.1 kg,0.5 kg, 1.0 kg and 5.0 kg, respectively. As a rough standard, a platecapable of withstanding a weight of 1.0 kg in this test would beacceptable for practical use of the reflecting mirror of 500 mmdiameter.

Example 2

The experimental procedure was substantially the same in Example 1except that expansion of the ammoniated and sintered body of the fusedquartz glass powder was performed at 1600° C. instead of 1500° C. Thethus obtained porous foamed body had a bulk density of about 0.1 g/cm³and about 75% by volume of the porosity was provided by closed cellshaving diameters in the range from 0.08 to 1.0 mm.

The results of the tests undertaken in the same manner as in Example 1were that the depression at the center of the base body was 0.8 μm underthe 500 kg weight and the residual depression after removal of theweight was 0.1 μm in Test 1 and the depression of the front plate at thecenter was 1.5 μm in Test 2. The result of Test 3 was that thedepression of the plate at the center was 0.1 mm, 0.3 mm and 0.6 mmunder the load of 0.1 kg, 0.5 kg and 1.0 kg, respectively, and crackswere formed in the plate by mounting a weight of 5.0 kg.

Example 3

The experimental procedure was substantially the same as in Example 1except that expansion of the ammoniated and sintered body of the fusedquartz glass powder was performed at 1450° C. instead of 1500° C. Thethus obtained porous foamed body had a bulk density of about 0.9 g/cm³and about 60% by volume of the porosity was provided by closed cellshaving diameters in the range from 0.01 to 0.8 mm.

The results of the tests undertaken in the same manner as in Example 1were that the depression at the center of the base body was 0.1 μm underthe 500 kg weight and substantially no residual depression was foundafter removal of the weight in Test 1 and the depression of the frontplate at the center was 2 μm in Test 2.

Comparative Example 1

The experimental procedure was substantially the same as in Example 1except that expansion of the ammoniated and sintered body of the fusedquartz glass powder was performed at 1700° C. instead of 1500° C. Thethus obtained porous foamed body had a bulk density of about 0.05 g/cm³and about 68% by volume of the porosity was provided by closed cellshaving diameters in the range from 0.08 to 3.0 mm.

The results of the tests undertaken in the same manner as in Example 1were that the depression at the center of the base body was 2.2 μm underthe 500 kg weight and the residual depression after removal of theweight was 1.2 μm in Test 1 and the depression of the front plate at thecenter was 0.8 μm in Test 2.

Comparative Example 2

The experimental procedure was substantially the same as in Example 1except that expansion of the ammoniated and sintered body of the fusedquartz glass powder was performed at 1400° C. for 3 hours instead of1500° C. for 2 hours. The thus obtained porous foamed body had a bulkdensity of about 1.2 g/cm³ and about 72% by volume of the porosity wasprovided by closed cells having diameters in the range from 0.1 to 0.6mm.

The results of the tests undertaken in the same manner as in Example 1were that the depression at the center of the base body was 0.1 μm underthe 500 kg weight and substantially no residual depression was foundafter removal of the weight in Test 1 and the depression of the frontplate at the center was 10 μm in Test 2.

Example 4

The experimental procedure was substantially the same as in Example 1except that expansion of the ammoniated and sintered body of the fusedquartz glass powder was performed at 1510° C. for 12 hours instead of1500° C. for 2 hours. The thus obtained porous foamed body had a bulkdensity of about 0.8 g/cm³ and about 30% by volume of the porosity wasprovided by closed cells having diameters in the range from 0.08 to 6.0mm.

The resets of Test 3 were that the depression of the plate at the centerwas 0.1 mm, 0.5 mm and 2.0 mm under the load of 0.1 kg, 0.5 kg and 1.0kg, respectively, and the plate was broken by mounting a weight of 5.0kg.

Example 5

The procedure for the preparation of a porous foamed body of fusedquartz glass was substantially the same as in Example 1 except that thesintering temperature of the fused quartz glass powder in the carbonmold was 1100° C. and the sintered and ammoniated silica body wasexpanded by heating at about 1800° C. for about 10 minutes to give aporous foamed body of fused quartz glass having a bulk density of 0.12g/cm³ of which the volume fraction of closed cells was 16% in theoverall porosity and the closed cells had diameters ranging from 0.1 to3.0 mm.

The thus prepared porous body was cut and shaped into a disc having adiameter of 1000 mm and a thickness of 49 mm and bonded to a front platehaving a diameter of 1000 mm and a thickness of 3 mm in the same manneras in Example 1 except that the temperature of the bonding work wasabout 1600° C. instead of 1300° C. to give a base body of a reflectingmirror. The surface of the front plate was ground and polished so thatthe finished front plate having an optically flat surface had athickness of 1 mm. Thereafter, aluminum was deposited on the thusobtained optical surface by the method of chemical vapor deposition togive a reflecting mirror which had a diameter of 1000 mm and a thicknessof 50 mm. The thus finished base body before deposition of aluminum onthe optical surface had excellent appearance without noticeable defects.The weight of the thus prepared base body was only about 6% of that of amirror base having the same dimensions but entirely formed from fusedquartz glass without porosity.

Example 6

The procedure for the preparation of a base body of a flat reflectingmirror was substantially the same as in Example 5 except that thesintered and ammoniated body of fused quartz glass powder was expandedat a temperature of 1700° C. for 15 minutes instead of 1800° C. for 10minutes so that the porous foamed body of fused quartz glass had a bulkdensity of 0.98 g/cm³ of which the volume fraction of closed cells was16% of the overall porosity and the closed cells had diameters rangingfrom 0.1 to 2.0 mm. The appearance of the thus prepared base body afterpolishing of the optical surface had appearance as good as in Example 5.

Example 7

The procedure for the preparation of a porous foamed body of fusedquartz glass was substantially the same as in Example 5 except that thesintering temperature of the fused quartz glass powder was 1300° C. andthe sintered and ammoniated body was expanded at a temperature of 1720°C. for 10 minutes instead of 1800° C. for 10 minutes so that the porousfoamed body of fused quartz glass had a bulk density of 0.8 g/cm³ ofwhich the volume fraction of closed cells was 60% of the overallporosity and the closed cells had diameters ranging from 0.02 to 1.0 mm.

The thus obtained porous body was cut and shaped into a disc having adiameter of 1000 mm and a thickness of 40 mm and bonded to a front platehaving a thickness of 12 mm in the same manner as in Example 5 to give abase body of a reflecting mirror which was subjected to grinding andpolishing of the surface of the front plate followed by deposition ofaluminum so that the front plate of the finished reflecting mirror had athickness of 10 mm.

Example 8

The procedure was substantially the same as in Example 5 except that thefront plate bonded to the porous foamed disc body had a thicknesssmaller by 1 mm than in Example 5 and a 1.0 mm thick plate of milkywhite fused quartz glass having a density of 2.2 g/cm³ was bonded to theother surface of the porous foamed disc body as a rear plate in the samemanner as in Example 1. The finished reflecting mirror had the samediameter and thickness as in Example 5. The weight of the thus preparedbase body was only about 8% of that of a mirror base having the samedimensions but entirely formed from fused quartz glass without porosity.

Comparative Example 3

The procedure was substantially the same as in Example 5 except thatsintering temperature of the fused quartz glass powder was 1000° C. andthe temperature for the expansion of the ammoniated and sintered bodywas 1760° C. so that the porous foamed body had a bulk density of 0.8g/cm³ of which the volume fraction of closed cells was about 10%.

The porous foamed body shaped into the form of a disc was bonded in thesame manner as in Example 7 to a front plate having a thickness of 12 mmso that the front plate of the finished reflecting mirror had athickness of 10 mm after grinding and polishing. The overall thicknessof the mirror was the same as in Example 5. The base body of thereflecting mirror before deposition of aluminum layer had a defect thatseparation, though very slight, was found between the front plate andthe porous foamed disc body.

Comparative Examples 4 and 5

The procedure for the preparation of a base body of a flat reflectingmirror was substantially the same as in Comparative Example 3 describedabove in each of Comparative Examples 4 and 5 except that the porousfoamed disc body had a bulk density of 0.05 g/cm³ and 0.1 g/cm³,respectively, the volume fraction of closed cells was each 15% of theoverall porosity and the front plate had a thickness of 1 mm and 0.5 mm,respectively, after finishing by grinding and polishing. The appearanceof the base body in Comparative Example 4 after polishing had a defectsimilar to that in Comparative Example 3 and a small number of tinycracks were found in the front plate in the base body of ComparativeExample 5 after finishing by polishing.

The reflecting mirrors prepared in Examples 5 to 8 and ComparativeExamples 3 and 4 were each subjected to the test of warping orundulation of the optical surface by the body weight when the mirror washorizontally supported at three symmetrical positions around theperiphery to determine the root mean square (RMS) roughness and themaximum height in the roughness curve (Rt) in the vertical andhorizontal directions. The results of the measurement are shown in Table1 below in the unit of λ, which was the wavelength 633 nm of the lightused in the measurement of the optical interference. The table alsoincludes the data obtained with a reflecting mirror as a control havinga honeycomb structure by sandwiching a latticework formed from fusedquartz glass pipes having a length of 46 mm, outer diameter of 40 mm andwall thickness of 2 mm in a closed-packing arrangement between two fusedquartz glass plates each having a thickness of 2 mm by welding into anintegral body. The weight of this control base body was 22% of that of amirror base having the same dimensions but entirely formed from fusedquartz glass without porosity.

                  TABLE 1                                                         ______________________________________                                                Vertical direction                                                                         Horizontal direction                                             RMS     Rt       RMS       Rt                                         ______________________________________                                        Example 5 0.06      0.24     0.07    0.28                                     Example 6 0.02      0.18     0.06    0.30                                     Example 7 0.03      0.11     0.04    0.28                                     Example 8 0.03      0.12     0.04    0.20                                     Comparative                                                                             0.03      0.14     0.14    0.92                                     Example 3                                                                     Comparative                                                                             0.09      0.82     0.66    3.55                                     Example 4                                                                     Control   0.07      0.25     0.12    0.66                                     ______________________________________                                    

Example 9

Silicon tetrachloride was subjected to flame hydrolysis in anoxyhydrogen flame by the chemical vapor deposition method to producesilica soot and a sintered body prepared therefrom was ammoniated byheating in an atmosphere of ammonia at 800° C. The thus obtainedsintered and ammoniated body of silica was heated at 1650° C. under areduced pressure of 0.1 Torr for 3 hours so that the silica body wassoftened and expanded by the gas evolved therefrom into a porous foamedbody of fused silica glass having a bulk density of 0.19 g/cm³, of whichthe volume fraction of closed cells was about 82% of the overallporosity. The closed cells had diameters in the range from 0.05 to 1.2mm. The porous foamed body of fused silica glass was cut and shaped intoa disc having a diameter of 506 mm and a thickness of 52 mm.

A plate of high-purity transparent fused quartz glass having a diameterof 500 mm and a thickness of 3 mm to serve as a front plate was mountedon the porous foamed disc body after spreading the same finely dividedsilica powder as used in Example 1 in an amount of 100 g/m² to form auniform layer of the powder and a graphite plate of 20 kg weight wasfurther mounted thereon. The assemblage was introduced into a furnaceand heated at 1400° C. for about 1 hour under a reduced pressure of 0.1Torr so that the silica powder of the interposed layer was softened andthe porous foamed disc body and the front plate were firmly bondedtogether into an integral base body. The density of the porous foameddisc body in the thus prepared base body had been slightly increased toabout 0.20 g/cm³ due to compression in the heating process at 1400° C.This base body was finished by grinding the side surface and grindingand polishing the surface of the front plate into a mirror base having adiameter of 500 mm and a thickness of 50 mm of which the thickness ofthe front plate was 1 mm.

The mirror base prepared in the above described manner was subjected tothe measurement of the peak-and-valley height deviation using an opticalinterferometer by holding the same to have the optical surface in ahorizontal and vertical dispositions. The results are shown in Table 2below in the unit of λ which was the wavelength 633 nm of the light usedin the interference measurement.

Example 10

A sintered body of silica soot prepared from silicon tetrachloride bythe chemical vapor deposition method was ammoniated by heating at 800°C. in ammonia and then subjected to expansion by heating at 1650° C. for3 hours under a reduced pressure of 0.1 Torr to give a porous foamedbody of fused silica glass having a bulk density of 0.19 g/cm³ and thevolume fraction of closed cells in the overall porosity was about 70%.The closed cells had diameters in the range from 0.05 to 1.3 mm. Thisporous foamed body of fused silica glass was cut and shaped into a dischaving a thickness of 52 mm and a diameter of 497 mm.

The porous foamed disc body was put into a circular hoop of fused quartzglass having an outer diameter of 504 mm, width of 48 mm and thicknessof 3 mm and the porous disc body was sandwiched between two circularplates of fused quartz glass to serve, one, as a front plate and, theother, as a rear plate each having a diameter of 502 mm and a thicknessof 3 mm by interposing a layer of the same finely divided silica powderas used in Example 1 between each surface of the porous disc body andthe glass plate in an amount of 100 g/m². The gap formed between thehoop and the porous disc body was filled with the same silica powder.This assemblage was mounted on a surface plate of graphite in anelectric furnace and pressed by mounting thereon a graphite block of 20kg weight to be heated in the furnace at 1400° C. for 1 hour under areduced pressure of 0.1 Torr so that the four parts were bonded togetherinto an integral base body. The thickness of the porous foamed disc bodyin the thus prepared base body had been decreased to 48 mm and thedensity thereof had been slightly increased to about 0.20 g/cm³ due tocompression in the heating process at 1400° C.

This base body was finished by grinding the side surface and grindingand polishing the surface of the front plate into a mirror base having adiameter of 500 mm and a thickness of 50 mm of which the thickness ofeach of the front plate and rear plate was 1 mm.

The mirror base prepared in the above described manner was subjected tothe measurement of the peak-and-valley height deviation in the samemanner as in Example 9 to give the results shown in Table 2 below.

Example 11

A porous foamed body of fused silica glass, of which the bulk densitywas 0.57 g/cm³ and the volume fraction of the closed cells havingdiameters of 0.02 to 0.8 mm was about 82% in the overall porosity, wasprepared in a manner similar to Example 10. The porous foamed body wascut and shaped into a disc having a diameter of 497 mm and thickness of52 mm. This porous foamed disc body was bonded to two plates andreinforcing hoop of fused quartz glass in the same manner as in Example10 to give a base body of a reflecting mirror. The thickness of theporous foamed disc body in the thus prepared base body had beendecreased to 48 mm and the density thereof had been slightly increasedto about 0.60 g/cm³ due to compression in the heating process at 1400°C.

This base body was finished by grinding the side surface and grindingand polishing the surface of the front plate into a mirror base having adiameter of 500 mm and a thickness of 50 mm of which the thickness ofeach of the front plate and rear plate was 1 mm. This mirror base wassubjected to the same measurements of the peak-and-valley heightdeviation in the same manner as in Example 9 to give the results shownin Table 2.

Comparative Example 6

A porous foamed body of fused silica glass was prepared in substantiallythe same manner as in Example 10 except that expansion of the sinteredand ammoniated body of silica soot was conducted at 1750° C. for 1 hour.The porous body had a bulk density of 0.045 g/cm³ and the volumefraction of closed cells having diameters of 0.1 to 6.0 mm was 14% inthe overall porosity.

This porous foamed body of fused silica glass was cut and shaped into adisc of 497 mm diameter and 52 mm thickness which was bonded togetherwith two plates of fused quartz glass and a reinforcing hoop in the samemanner as in Example 10 into an integral base body except that thegraphite weight of 20 kg was replaced with that of 8 kg and heating at1400° C. in an electric furnace was performed for 30 minutes instead of1 hour. The thickness of the porous foamed disc body in the thusprepared base body had been decreased to 48 mm and the density thereofhad ben slightly increased to about 0.050 g/cm³ due to compression inthe heating process at 1400° C.

This base body was finished by grinding the side surface and grindingand polishing the surface of the front plate into a mirror base having adiameter of 500 mm and a thickness of 50 mm of which the thickness ofeach of the front plate and rear plate was 1 mm. This mirror base wassubjected to the same measurements of the peak-and-valley heightdeviation in the same manner as in Example 9 to give the results shownin Table 2.

                  TABLE 2                                                         ______________________________________                                                Vertical disposition                                                                       Horizontal disposition                                   ______________________________________                                        Example 9 0.15 λ  0.26 λ                                        Example 10                                                                              0.12 λ  0.17 λ                                        Example 11                                                                              0.10 λ  0.15 λ                                        Comparative                                                                             0.50 λ  0.95 λ                                        Example 5                                                                     ______________________________________                                    

What is claimed is:
 1. A base body of a reflecting mirror which is anintegral body comprising:(A) a front plate having an optically flat orcurved surface made from transparent fused quartz glass or high-silicaglass; and (B) a porous foamed body of fused quartz glass or high-silicaglass bonded to the surface of the front plate opposite to the opticallyflat or curved surface, wherein the porous foamed body has a bulkdensity in the range from 0.1 to 1.1 g/cm³, and at least 30% by volumeof the porosity of the porous foamed body consists of closedcells,wherein the porous foamed body is bonded to the front plate with alayer of a melt of a finely divided silica powder.
 2. The base body of areflecting mirror as claimed in claim 1 in which the front plate has athickness in the range from 2 to 20% of the total thickness of the frontplate and the porous foamed body.
 3. The base body of claim 1, whereinthe porous foamed body and front plate are bonded with the layer ofsilica powder by heating an assembly thereof at 1400° to 1600° C. atabout 1 to about 4 hours.
 4. The base body of claim 1, wherein at least60% of the porosity of the porous foamed body is provided by closedcells.
 5. The base body of claim 1, further comprising a rear plate offused quartz glass or high-silica glass bonded to the surface of theporous foamed body opposite to the front plate.
 6. The base body ofclaim 5, further comprising a reinforcing layer surrounding and bondedto the side of the porous foamed body.
 7. A base body of a reflectingmirror which is an integral body comprising:(A) a front plate having anoptically flat or curved surface made from transparent fused quartzglass or high-silica glass; and (B) a porous foamed body of fused quartzglass or high-silica glass bonded to the surface of the front plateopposite to the optically flat or curved surface, wherein the porousfoamed body has a bulk density in the range from 0.1 to 1.1 g/cm³, andat least 30% by volume of the porosity of the porous foamed bodyconsists of closed cells having a diameter of 0.01 mm to 3 mm,whereinthe porous foamed body is bonded to the front plate with a layer of amelt of a finely divided silica powder.
 8. The base body of claim 7,wherein the porous foamed body and front plate are bonded with the layerof silica powder by heating an assembly thereof at 1400° to 1600° C. atabout 1 to about 4 hours.
 9. The base body of claim 7, wherein at least60% of the porosity of the porous foamed body is provided by closedcells.
 10. The base body of claim 7, further comprising a rear plate offused quartz glass or high-silica glass bonded to the surface of theporous foamed body opposite to the front plate.
 11. The base body ofclaim 10, further comprising a reinforcing layer surrounding and bondedto the side of the porous foamed body.
 12. The base body of a reflectingmirror as claimed in claim 7 in which the front plate has a thickness inthe range from 2 to 20% of the total thickness of the front plate andthe porous foamed body.